All you need is to have a check at the top of your verbs to see if you can teleport it or not.
For example:

var/turf/T = locate(usr.x,usr.y,usr.z) //Put the turf under the player into a varif(!T.CanTeleport) //Check if the CanTeleport var of the turf is false usr << "You can't teleport here!"return//End the verb here, don't do anything else

Of course, that would require that /turf had a CanTeleport variable. You'd also have to change it for the turfs you want to not be able to teleport from.